Further Studies of DNA Damage and Lethality from the Decay of Iodine-125 in Bacteriophages

Summary

The DNA of coliphages T4 and T1 was labelled with ¹²⁵I-iododeoxyuridine. ¹²⁵I decay is known to cause severe molecular damage via vacancy cascades (the Auger effect). We have compared the induction of both single- and double-strand breaks (SSBs and DSBs) in ¹²⁵I-labelled T4 DNA stored at −196°C during decay, either as intact phage or as free DNA. These comparative experiments indicate that, in addition to one DSB which apparently results directly from the Auger effect, each decay in an intact phage also give rise to an additional 0·05 DSBs, as well as 1·6 SSBs, as a result of ionizing radiation absorbed in the same phage particle where the decay occurs. An examination of T4-killing by ¹²⁵I decay reveals a two-phase survival curve, whose initial slope corresponds to a lethal efficiency per ¹²⁵I decay of 0·95 ± 0·05, which is considerably higher than values previously determined. The results for phage T4, and of a more limited comparison of ¹²⁵I suicide and DNA damage in phage T1, support the hypothesis that the vacancy cascades which accompany each ¹²⁵I decay in DNA result in a double-strand break at the decay site and that each such break is a lethal event.